This invention relates to titanium alloy/fiber composite materials. In particular, this invention relates to a method to produce high temperature oxidation resistant composite materials.
Composites are recognized as a material class capable of operating under conditions requiring very high specific stiffness and strength. Synthetic matrix composites are generally limited to maximum operating temperatures of about 200.degree. C. Metal matrix composites are capable of higher operating temperatures. Aluminum- and titanium-based composites comprise the majority of metal matrix composites employed, particularly in aerospace applications.
Titanium composites are fabricated by several methods, including superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers by vacuum hot pressing, hot isostatic pressing, and the like. At least four high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide. Under superplastic conditions, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus--contacting layers of matrix material bond together by a phenomenon known as diffusion bonding. Unfortunately, at the same time a reaction occurs at the fiber-matrix interfaces, giving rise to what is called a reaction zone. The intermetallic compounds formed in the reaction zone may include reaction products like TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2. The thickness of this brittle reaction zone is a diffusion controlled reaction and thus increases with increasing time and with increasing temperature of bonding. Such brittle reaction zones introduce sites for easy crack initiation and propagation within the composite, which can operate in addition to existing sites introduced by the original distribution of defects in the filaments and/or the matrix.
Aluminum-based composites are currently limited in application to about 800.degree. F., due to their degraded matrix strength at higher temperatures. Titanium- and nickel-based metal matrix composites are currently considered for many advanced aerospace applications such as airframes and high compression gas turbine engines at temperatures as high as 1600.degree. F. (870.degree. C.).
Research on the effects of prolonged high temperature exposure to air or an oxidizing environment has shown that metal matrix composites may suffer severe loss of strength, fatigue and creep resistance due to oxygen diffusion from the component surface into the fiber/matrix reaction zones nearest the surface. The reaction zone can, to some extent, be controlled by providing the fibers with a barrier coating, incorporating reaction zone reducing elements into the matrix, control of fabrication conditions, or the like. Oxygen diffusion into the composite can embrittle the reaction zone and/or damage the fiber, leading to early fiber fracture by tensile, creep, impact or fatigue loading.
The stiffness (E.sub.c) and tensile strength (.sigma..sub.c) of metal matrix composites are calculated using the rule-of-mixtures (ROM) formulae:
______________________________________ Stiffness (E.sub.c): E.sub.c = E.sub.f (V.sub.f) + E.sub.m (1 - V.sub.f) Longitudinal Tensile Strength (.sigma..sub.c): .sigma..sub.c = .sigma..sub.f (V.sub.f) + .sigma..sub.m '(1 - V.sub.f) ______________________________________
where E.sub.f is the fiber modulus, E.sub.m is the matrix modulus, V.sub.f is the fiber volume, .sigma..sub.f is the fiber tensile strength and .sigma..sub.m ' is the matrix stress when the fibers are at their ultimate tensile strain. Thus, oxygen diffusion into the composite can reduce the effective volume fraction of fibers by destroying the fibers and/or by embrittling the interface between the matrix and fiber. According to the above formulae, the composite stiffness and tensile strength are correspondingly reduced.
Accordingly, it is an object of this invention to provide a method to produce improved high temperature oxidation resistant titanium alloy matrix composites.
Other objects and advantages of the invention will be apparent to those skilled in the art.